The present invention relates to an optical head apparatus for optical discs, and particularly to an optical head used for producing master optical discs.
In order to increase recording density of an optical disc, laser light having a short wavelength and/or an object lens with a large numerical aperture must be used.
However, it is not easy to develop a semiconductor laser which emits laser light having a short wavelength. While enlarging the diameter of the object lens broadens the numerical aperture of the object lens, the optical head itself consequently becomes large in size.
Hence, recording information on an optical disc using a Solid Immersion Lens (referred to as SIL, hereinafter) is proposed by Mr. S. M. Mansfield et. al. in "High-numerical-aperture lens system for optical storage" (OPTICS LETTERS/Vol. 18, No. 4 305-307/Feb. 15, 1993).
This is a method wherein laser light gathered by the object lens is radiated on the SIL, which further converges the laser light. A minute spot can be formed by using the SIL.
In order to form a minute spot by the SIL, it is necessary to converge the laser light onto the bottom surface of the SIL. Therefore, the laser light must be led into the SIL after having been sufficiently gathered by the object lens. In other words, the object lens must be controlled so that it can be always optimally positioned relative to the SIL.
In order to perform the above-mentioned control, it is necessary to generate a focus error signal according to the reflected light from the SIL. However, the reflected light detected by a detector includes those reflected from the spherical surface of the SIL, the bottom surface of the SIL, and the master optical disc or the like.
Hence, the output derived from the light received by the detector includes much noise and is not acceptable to be used directly as the focus error signal for controlling the object lens to be optimally positioned.